Well, you could agitate with your local lawmakers to limit parking to a certain number of hours, or even to ban it completely on the grounds that the street is too narrow. If his parking is within the limits of current law, there really is no right way to approach him without appearing unreasonable, because your desire is inherently unreasonable. Asking someone to limit their legal use of a public resource is really out of line.
If your could articulate some concrete benefit -- like you wanted the spot for your own car, which has to be *somewhere* on the street so it might as well be there -- then you could simply ask for the favour and owe him one. Do not imagine that concern for the elderly neighbour and the letter carrier represents a real benefit; these are shams to cover up for your control needs. The Post Office is finding your mailbox just fine (hint: it's their job, and there's harder ones than yours), and if granny can't back out of her driveway without being a hazard, it's time to take her license away.
But really, Susie, you need to move to a nice controlled community, like a condo development or a homeowner's association area, where the rulemakers think like you. In fact I suspect you will find yourself welcomed into the ranks of the governing committees. Like many people, you will never be happy in a free society. After you bully your neighbour into ceasing this perfectly legal behaviour you will find something else to complain about.
The car, by the way, is a classic, a more sensible person would be tickled to see it.
S.